Shortly after his election in November, Donald Trump promised that within his first 100 days in office he would slash a host of restrictions on the American energy industry, unleash the potential of shale gas and "clean coal" and create "millions of high-paying jobs."

His America First Energy Plan, posted to the White House website just after he took the oath of office, was similarly effusive about the role fossil fuels would play in America's future energy mix. The Trump administration was committed to clean coal technology, it says, "and to reviving America's coal industry."

The rhetoric played well with voters in the coal-producing states of Appalachia and the Powder River Basin in the northern Great Plains.

Now, a similar push is taking shape on the other side of the world.

Australia is the world's largest coal exporter, according to the International Energy Agency, and one of the planet's biggest carbon emitters on a per capita basis, largely due to its reliance on coal-fired power plants. Its conservative Coalition government, led by Prime Minister Malcolm Turnbull, has begun touting the benefits of clean coal, too. Given the intermittent nature of renewable energy production, the government says, coal is essential for Australia's energy security.

Coal is the most carbon-intensive source of power generation. Emissions from burning it help fuel global warming that's melting polar ice caps and causing more frequent, more intense weather extremes, like downpours and droughts. Coal burning also emits sulfur dioxide, nitrous oxides, particulate matter, and mercury, which are tied to respiratory illnesses, acid rain, smog and haze, and neurological problems in humans and animals.

When scientists talk about clean coal they are usually referring to two methods: High-efficiency, low-emissions technologies and carbon capture and storage.

HELE technology most commonly refers to coal-fired combustion that produces steam at much higher temperatures and pressures than in conventional electricity generation. Higher temperatures mean greater efficiency, and less coal used per unit of electricity generated.

But environmental groups and some scientists say labeling HELE technology as clean coal is misleading.

"I think we need to be careful with that term - high-efficiency, lower-emissions is reasonable," said Dr. Iain MacGill, an Associate Professor in the School of Electrical Engineering and Telecommunications at the University of New South Wales.

CSS technology, meanwhile, involves capturing carbon dioxide emissions either before coal is burned or from smokestacks after combustion, then pumping it deep under ground.

Although decades old, investment in CSS technology has gained momentum over the past several years. Around the world there are 38 large-scale projects either in production, under construction or being planned, according to a report released by the Global CSS Institute in 2016.

Energy experts say the technology will be crucial in limiting global greenhouse emissions to no more than 2 degrees above pre-industrial levels by the end of the century.

But despite its promise, the technology has drawbacks; the biggest of which is the high price tag.

"It is a significant amount of money that needs to be deployed on individual projects and that has a risk aspect," said Dr. Daniel Roberts, an energy expert at CSIRO, Australia's national science agency.

The high costs of deploying carbon capture technology, building vast pipelines to transport CO2 and then injecting it into the ground have proven a turnoff for investors, especially if they face policy uncertainty, like a stable price on carbon emissions.

Rapidly falling prices for manufacturing, installing and producing renewable energy, like wind and solar, is also undermining its commercial viability.

The U.S. now has one large "clean coal" plant in operation out of more than 400 that are coal-fired.

The $1 billion Petra Nova plant in Texas began production in January and traps about 90 percent of its carbon dioxide emissions. Another, the Kemper plant in Missisipi, however, is running two years behind schedule and is more than $4 billion over budget.

Investment in large-scale carbon capture projects in Australia has been even less successful. Only the Gorgon LNG Project, off northwest Australia, is using the technology, although the government has said it will look at funding clean coal-fired power plants.

Despite the lack of widespread deployment, proponents say the technology has an important role to play.

"For as long as we choose to use fossil fuels, whether in industrial process or energy production, we're going to have to use CSS," Professor Peter Cook, a CSS expert at the University of Melbourne in Australia, said.